Laravel MongoDb Aggregation For unique and null

I want to retrieve documents from mongoDb by using MongoClient for parent_id is only first of same and parent_id is null.Only one aggregation.

$search[] = [
               '$group' => [
                    '_id' => '$parent_id',
                    'doc' => ['$first' => '$$ROOT']
                ]
            ];
            
            $search[] = [
                '$replaceRoot' => ['newRoot' => '$doc']
            ];

 app('mongo')->todo->Product->aggregate(array_filter($search));